The invention relates generally to hydrostatically supported rolls and, more particularly, to an improved internal support element for such rolls.
Support elements for hydrostatically supported rolls are disclosed in German Patent No. 2230139. The support elements of this patent have a cylindrical contact surface that conforms to the inner circumference of the hollow cylinder for transmitting contact pressures, which act radially against the inner circumference of the hollow cylinder, via a load bearing liquid film. This liquid film is formed by hydraulic fluid, which constantly flows from supply pockets formed between segments on the top side of the support elements to avoid metallic friction between the contact surface and inner circumference of the cylinder.
When friction normally occurs between two solid surfaces, the attendant frictional forces and, hence, the dissipated energy that is converted into heat, are independent from the surface to which the contact pressure is transmitted. On the other hand, with roll support elements, fluid friction develops in the gap formed between the inner circumference of the hollow cylinder and the contact surface on the top side of the support elements. The fluid friction arises from the friction of the liquid in the gap, which is subjected to high pressure and high shear stress. This friction is dependent upon the extent of the gap, i.e., the circumferential extent of the segments formed on the top side of support elements, which define the contact surface. The greater the circumferential extent of the segments, the greater the length over which the liquid experiences friction and, therefore, the greater is the amount of frictional heat that is generated in the gap. The fact that a relationship exists between the size of the segments of the support elements and the generation of heat, which is dependent upon the fluid friction, is disclosed, per se, in German Published Patent Application No. 3611858. This relationship is not readily observed when the circumferential speed of the hollow cylinder is insignificant relative to the support elements. However, if the circumferential speed increases, a significant amount of power develops from the friction generated at each support element. With standard support element dimensions and a contact surface diameter of approximately 100 to 200 mm, this friction power can have a magnitude of 1 kW (kilowatt) and above, depending upon the circumferential speed of the hollow cylinder. When friction power of this magnitude is generated nonuniformly over the length of the support elements, due to the variable circumferential extent of the segments of the support elements, noticeable temperature differences occur along the outer circumference of the hollow cylinder. This temperature differential may have detrimental effects during certain treatments of paper webs, nonwoven fabrics or the like.